A position of a tumor which is to be irradiated on radiation therapy will move by respiration and daily variation or deformation of an organ. In general, X-ray transmission images (two-dimensional images) are used to conduct registration so that an organ to be irradiated (a target) can be placed at a predetermined position on radiation therapy.
For example, regarding an organ which moves in association with respiration, a respiratory gating method is used to give irradiation only to a region where a respiratory waveform is found, thereby reducing redundant irradiation occurring by respiratory motion.
However, this respiratory waveform represents a one-dimensional position at a certain point established on the body surface, and an organ also moves three-dimensionally during irradiation. Therefore, a more accurate irradiation is required to ascertain the motion of the organ at individual positions (the speed and direction of movement), the analysis of which is, however, difficult by referring to two-dimensional images.
In particular, where a lung at which a tumor and normal tissues are greatly different in density is subjected to particle beam therapy, the variation of the organ not only in a direction perpendicular to the irradiation beam (herein after simply referred to as beam) but also in an axial direction of the beam greatly influences a range of the beam, thus, resulting in a fear that the tumor may be irradiated to an excessively small extent or normal tissues may be irradiated to an excessively great extent. Therefore, a technology to three-dimensionally ascertain the motion of an organ is now demanded. However, such a technology has not been established with current clinical practices.
In recent years, several reports have been submitted about research of quantification of positions of an organ three-dimensionally. Most of the reports are those in which a reference point which is a marker for tracking movement is given on the surface of a target organ extracted by using X-ray CT images, and the deformation is calculated by referring to the organ as an elastic body model so that reference points on two CT images can coincide with each other.
However, in this instance, it is necessary to set a feature point artificially. Matching of each position inside an organ does not necessarily reflect actual movement. Furthermore, in order to track a specific movement, a metal marker is used in general. The metal marker is, however, implanted into the body of a patient, thus giving a greater burden to the patient. The marker is also restricted in the position and number, thereby making it difficult to ascertain the motion of an organ in detail.
T. Zhang et. al., “Technical note: A novel boundary condition using contact elements for finite element based deformable image registration,” Medical Physics 31 (2004) 2412-2415 (hereinafter referred to as Non-patent document 1) has described that a finite element method is used to calculate a three-dimensional deformation of a lung.
Furthermore, D. Sarrut et. al., “Non-rigid registration method to assess reproducibility of breath-holding with ABC in lung cancer,” International Journal of Radiation Oncology, Biology, Physics, 61 (2005) 594-607 (herein after referred to as Non-patent document 2) has described that gradients of voxel values obtained by three-dimensional CT images of the lung are compared to quantify the deformation inside the organ and also discussed the deformation of volume of interest at a tumor portion.
Still furthermore, Hidenori Shitaka, et. al., “An algorithm for localizing branchpoints of pulmonary vessels for non-rigid registration of the lung” The Institute of Electronics Information and Communication Engineers D-II, Vol. J84-D-II, No. 07 pp. 1-11 (herein after, referred to as Non-patent Document 3) has described the tracking of movements at vessel bifurcations for matching inside the organ with the deformation of a lung in association with respiratory motion.
However, the technology described in Non-patent Document 1 has a problem that the outer configuration of an organ does not necessarily coincide with the deformation inside the organ.
Furthermore, the technology described in Non-patent Document 2 does not track an anatomical feature inside an organ, thereby posing a problem regarding the accuracy of volume-of-interest deformation.
Still furthermore, the technology described in Non-patent Document 3 is only to position vessel bifurcations.